The proposed project is a continuation of studies aimed at elucidating the factors responsible for the decreased immunogenicity of tumors and normal tissues which have been explanted by the organ culture technique. Such explants often are transplantable across strong histocompatibility barriers. The subsequent changes in the immune reactivity of the allograft recipients lower their ability to reject non cultured allografts syngeneic to the explanted tissue. The nature of the host changes and methods of reversing them (to induce rejection) are being studied. Indications that the lymphoid tissues from the allograft recipients behave more like those of sensitized individuals than tolerant ones will be explored using mixed lymphocyte cultures, cytotoxicity tests and tracing of radioactively labeled cells in vivo.